In refinery of non-ferrous metals, mercury contained in the raw sulfide ore or in the slag in small amount will be turned out in a form of compound or metal vapour by thermal treatment procedures, such as heating, roasting and so on, and will be included in gases.
Since the vapour pressure of mercury is relatively high, it is often contained in the exhaust gas in gaseous or finely dispersed state, and therefore, if these gases are exhausted without treatment, the atmosphere will be polluted when such gases are used for the production of sulfuric acid, the thereby obtained sulfuric acid will be contaminated with mercury due to absorption.
In general, for removing metals or its compounds in gases, there have been many procedures, which may be employed separately or in combination, for example, using scrubber or bag filter, cyclone separation, gravity setting and Cottrell precipitation. While it will be possible to remove, for example, mercury compounds in finely dispersed state by these procedures, they cannot attain a complete removal of gaseous substance, such as mercury vapour, since all these procedures are based on a physical dust removal principle.
On the other hand, in order to remove small amount of mercury in chemical way, there are several methods proposed. For example, a solution having oxidizing property, such as a solution containing sulfuric acid, is used for scrubber liquid, and a chemical reaction with iodide ion etc. is utilized. In these methods, however, the effectiveness of mercury removal by oxidation will vanish by reduction by sulfurous acid gas for example, so that they cannot be assumed as excellent for removing mercury in gases from the thermal treatment prosesses, such as heating, burning and so on in refining non-ferrous metals.
For removing mercury there has further been proposed a method, in which the gases are passed through an aqueous solution containing a substance which absorbs mercury easily, such as thiourea or so on. However, in this method, the pressure loss is considerably large due to the forced passage of gases through the aqueous solution, so that this is problematic in the apparatus.